ads124x_convert: wait for DRDY  before writing/reading the SPI bus

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Some cleanup

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Add function to set sample rates

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>